Workflow Question: How to get from a 3D scan to Aurasma Augmented Reality
Decimate modifier doesn't take in account UV maps.

With the collapse option enabled it:


  ...merge (s) vertices together progressively, taking the shape of the mesh into account


While mergin vertex, it likely happens that the UV mess up. You should create a new UV map for the decimated object and stick the original texture to it.


Start from duplicate (Shift+D) your mesh. Rename the source object
"Highpoly"    and the duplicated one "Lowpoly" (keep them in place,
overlayed)
Add and apply a Decimate modifier with the wanted settings on the Lowpoly object
The UVmap of the Lowpoly has now been destroyed in the decimation, you must re-unwrap your model.
Bake the original texture from the Highpoly to the Lowpoly as shown for example in this video (the only difference is that you should bake "textures" instead of "normals")